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Cobra 148 GTL mod

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  #41  
Old 04-24-2011, 09:42 AM
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Has anyone done the clarifier mod on the newer China made 148 GTL ? I would like to open mine up to slide on TX. Can anyone help me with this mod ? Any help here will be greatly appreciated. the 8 volt line on the clarifier is BLACK, with 2 RED wires on the other terminals. thanks for reading this post, hope to hear from someone on this, 73.
Dave
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  #42  
Old 04-24-2011, 11:36 AM
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Quote:
Originally Posted by psycho View Post
Unless you are using a freq counter, don't unlock the clarifier. The clarifier will zero at different locations on the dial when the ambient temperature changes.
As far as R131 goes, that'll only open up AM....there is a variable pot on the board to open up the ssb. Cobra 148s sound great with the limiter clipped.....most exports don't.
Who keeps bringing these old posts back to life??

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  #43  
Old 04-18-2013, 05:38 PM
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Quote:
Originally Posted by Toll_Free View Post
Cutting out D52 disables the radios internal VR that controls TX frequency. You CANNOT have it in, and have an unlocked clarifier.

The simple fact that you can slide a complete channel means something is fubar.

You do NOT want to ground one side of the clarifier control. THAT is going to get you the massive amount of slide, or screwing with the voltage divider around the clarifier... Which is what has happened here. We need to get the voltage divider set back up correctly, and then you'll be back 'stock', but with tx tracking the same as Rx.

I'm gonna GUESSTIMATE that the 800 ohm resistor is still tied to RX only voltage. One end should go to the junction I pointed out earlier, then the OTHER end should go to the full time 8 volts. If it's still on part time, so to speak, then the voltage divider is NOT going to function correctly.

Shorting out R174 will ground nothing. That's the resistor in series with the potentiometer, at least on the two schematics I'm looking at now. Do NOT SHORT THIS RESISTOR OUT!!!!!

It also sounds like you either lifted the wrong end VR402.

One side of R44 is tied to 8 volts, BUT ONLY IN RX!!!. This needs to be changed to the TX/RX voltage. Depending on HOW you performed the mod to change the voltage, you may or may not have removed that from the switched voltage. R44 is SUPPOSED to be 8200 ohms, btw.... At least on the schematic I have, it's marked 8k2. On coding method of resistances in the form of 4k7, 8k2 or 560R?, they give that as 8200 ohms, which is how I've been interpreting it for years.... BUT, we'll go on.

If you have a schematic, and can read it (I was assuming you couldn't), then I can give you another way to perform the mod..... You can work it out by looking at the schematic and the mod, then apply it 'in effect' on the radio.

The junction of RV104 and R44 is supposed to go to +8 volts ALL THE TIME.

Then remove D52. You're done.

If you're an entire channel off and get no slide (or voltage change), it sounds like you screwed up and pulled the WRONG side of the RV104... If BOTH sides are at 8 volts, it's going to read 8 volts no matter what you do..... One side has to be lower (closer to ground) than the other.

Jumpering or changing ANY of the following is going to SCREW your clarifier up, and make it NOT track like stock.

R175
R174
R44

Kill d52. Take the red wire from the clarifier and put it on a CONSTANT 8 volts. Go to R44. One side will join R174 and D51. Pull the OPPOSITE side and connect it to constant 8 volts (the SAME connection!!!!, preferably, connect it to the hot side of the RV104).

You're doneski. If you change ANY of the resistors above, you screw up the voltage divider in the radio.

I know I've repeated myself, but I tried to explain it a couple different ways. Grab a decent schematic you can blow up online, and I think it will make sense... You seem to have a basic grasp of electronics, so I'll give ya an idea of how it works.

The clarifier changes voltage on a diode that changes capacitance with a change in voltage. It is NOT a linear change, meaning at the high and low ends of the voltage range, things change VERY quickly!!!! Leading to the problems with clarifiers being WAY too touchy!!

They limit this effect by operating the diode in it's 'linear range', meaning if 1 volt = 1 pf of change from 3 to 6 volts, THATS where they are going to design the clarifier control to operate.... Even though it's connected to 8 volts on one side, and ground on the other. They form a resistive divider that allows your actual clarifier control to ONLY change from the 3 to 6 volts (and these are numbers pulled outta you know where.... I'm not positive what the range on the clarifier control is, and not wanting to do the math right now lol).

Hope this makes sense. I'm GLAD to see you actually did it, and then are willing to see if you can't make it work. BUT, I think you have R44 wrongfully placed (which I didn't make it clear before that you needed to make sure it was on the now constant voltage) or you pulled the WRONG wire from the clarifier. Just make SURE it's the red wire that goes to 8 volts now, and that you lift the 8 volt switched side of R44 and put it to the same connection as the red wire. Then clip D52 and you're done, center should be center, and you'll have stock slide.

--Toll_Free
hi r44 shows voltage drop on both sides when mic is keyed

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  #44  
Old 04-18-2013, 05:40 PM
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on my radior44 is showing 0 voltage when mic is keyed.

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  #45  
Old 04-18-2013, 08:52 PM
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One side of R44 is tied to 8 volts, BUT ONLY IN RX!!!. This needs to be changed to the TX/RX voltage. Depending on HOW you performed the mod to change the voltage, you may or may not have removed that from the switched voltage. R44 is SUPPOSED to be 8200 ohms, btw.... At least on the schematic I have, it's marked 8k2. On coding method of resistances in the form of 4k7, 8k2 or 560R?, they give that as 8200 ohms, which is how I've been interpreting it for years.... BUT, we'll go on.



in a stock clarifier the 8 volts that the pot varies is only available in RX, thats why you see the voltage drop off in TX.

the point of the mod is to lift the RX only 8 volts and provide a constant 8 volts to the clarifier.
pin 1 of the MB3756 chip is a good place to get this from.
LC
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  #46  
Old 04-18-2013, 09:37 PM
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Tag, good info here.



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